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XV. Experiments on the friction and abrasion of the surfaces of solids. By 
George Rennie, Esq. F.R.S. 
Read June 12, 1828. 
The paper now offered to the consideration of the Royal Society, comprises 
the results of part of a series of experiments undertaken in the year 1825, with 
a view to determine the measure of the retardations of bodies in motion, when 
affected by the attrition of their surfaces, and by mediums of different densities. 
From the attention that has hitherto been paid to this important branch of 
mechanical science, and from the many elaborate dissertations and experiments 
that have appeared at different periods, it would naturally be concluded, that 
the subject had been so fully elucidated, as to admit of little if any further in- 
vestigation : but the diversity of opinions still prevalent among philosophers, 
and the difficulty of reducing to a satisfactory state the doctrines already ad- 
vanced, incline me to the opinion that the subject is as yet but imperfectly 
understood. This may be attributed in a great degree to the very defective 
state of our knowledge of the properties of materials, and the difficulty or 
rather impossibility of subjecting them to geometrical mensuration. The 
science of mechanics considers forces as reduced to the simple questions of 
mathematical analysis, without regard to the properties of matter or the phe- 
nomena incident thereto : but in rendering forces sensible, we are necessarily 
compelled to make use of agents, or intermediate bodies termed machines, the 
employment of which in transmitting motion, in modifying its action, or in re- 
storing the equilibrium between forces of different intensities, constitutes the 
object of every mechanical operation. The solution of this question therefore 
involves the conditions of equilibrium, both of simple and compound machines; 
the transmission of motion under different circumstances; the construction and 
combination of the different parts of machines, and the properties of the ma- 
terials of which these parts are composed. 
